1. Field of the Invention
This invention relates to a method of migrating a melt of a metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) and, in particular, to the uniform initiation of promotion of reasonable rates of thermal migration by utilizing an element to enhance the penetration of a melt into the material at the surface and to function as a carrier droplet for a dopant or a life-time adjusting material which has either a high vapor pressure and/or a slow rate of migration.
2. Description of the Prior Art
W. G. Pfann in U.S. Pat. Nos. 2,739,088 and 2,813,048 describes methods of migrating melts of metal through particular regions of a solid body of semiconductor material by thermal gradient zone melting. However, molten line and droplet stability resulted in the breakup of the migrating lines and droplets and consequently acceptable semiconductor devices were not always obtainable.
Recently, we discovered that preferred planar orientation of the surfaces of the body of semiconductor material, migration axis and line orientation axis relationship were also a necessity to migrate liquid metal wires and/or droplets through the solid body. (See U.S. Pat. Nos. 3,899,362 and 3,904,442 for example.) These improvements in TGZM resulted in commercialization of the process. However, as the width of the lines being migrated became smaller, the penetration of fine liquid lines of less than 2 mils in width, and preferably 1 mil in width, and small liquid droplets, less than 6 mils in diameter, from the surface of a wafer or body of semiconductor material has been difficult to achieve repeatedly, on a commercial basis, by a thermal gradient alone. Although a thermal gradient is strong enough to cause migration of the small liquid zones once they are formed in the bulk of semiconductor material, the thermal gradient force is not powerful enough to overcome the surface tension forces holding fine liquid zones, or wires, on the surface of a body, or wafer. Further improvements to the TGZM processing techniques included alloying the deposited metal to the surface (U.S. Pat. No. 3,897,277) and sintering of the same (U.S. Pat. No. 4,006,040). The problem still persists as one attempts to migrate fine lines and droplets on a commercial basis. As a result, TGZM to date has been limited to line and droplet dimensions typical of solid-state power devices and has not had any commercial impact on integrated-circuit type devices which require a much finer size of doped region.
Although aluminum has been the predominant metal migrated by TGZM one needs other configurations than P+N type diodes and the like. However, some other elements used in doping semiconductor materials either have too high a vapor pressure or too slow a rate of migration when employed in TGZM processing.
Improvements in processing semiconductor material can be obtained if the melt penetration into the material prior to thermal migration could be enhanced by including a material therein, along with the one or more dopant impurity materials, which would prevent premature vaporization and/or improve the rate of thermal migration through the semiconductor material.
Therefore, it is an object of this invention to provide a new and improved method to migrate molten lines and droplets of metal through a solid body of semiconductor material by thermal gradient zone melting (TZGM) processing which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved method for initiating the penetration of a melt to be migrated thermally regardless of whether it is for fine molten lines and droplets or larger.
A further object of this invention is to provide a material in the melt to be migrated thermally through the semiconductor material which will enable one to achieve reasonable rates of thermal migration and/or uniform penetration of the melt into the material prior to initiation of migration.
Other objects of this invention will, in part, be obvious, and will, in part, appear hereinafter.